Adapt Xvfb instruction docs to different init systems in our images

[iriberri]

At https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-to-run-tests-that-require-a-gui, we're documenting the new services: xvfb method but we still mention starting xvfb using the /etc/init.d/xvfb file, which does no longer exist in the Xenial env that uses systemd.

I see that these two sections:

• https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-xvfb-directly
• https://docs.travis-ci.com/user/gui-and-headless-browsers/#starting-a-web-server

still mention /etc/init.d/xvfb. The "Starting a web server" one seems easily adaptable by adding to it to new services: xvfb approach. For "Using Xvfb directly", should we split the section with the upstart/systemd approaches? What do you think @joepvd?

Code context: https://github.com/travis-ci/travis-build/blob/664f39d1c7da26e145a85974d71d7972bf2588fc/lib/travis/build/appliances/services.rb#L82-L90

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